There have been a number of proposals to form an intake manifold by molding plastic material primarily for the purpose of minimizing the weight of the engine. For instance, Japanese patent laid open publication (kokai) No. 2001-342918 discloses an intake manifold consisting of two pieces that are bonded to each other along a plane in parallel with the direction of the intake flow to define individual intake pipes that are commonly connected to a surge tank.
A number of dual port intake manifolds have also been proposed. According to the proposal disclosed in Japanese utility model publication (kokoku) H03-41056, each individual intake pipe includes a pair of intake pipe sections having different lengths branching off from the surge tank and connected to a same cylinder in a mutually parallel relationship and a butterfly valve for selectively closing one of the intake pipe sections depending on the load condition of the engine for the purpose of optimizing the intake volumetric efficiency of the engine over a wide operating range of the engine.
However, the aforementioned proposal to form an intake manifold by injection molding plastic material is not suitable for molding an intake manifold having a more complex configuration as is the case when the intake manifold is to be used as a part of a dual port intake device because of the limited freedom in design. In particular, the bearing arrangement for the selection valve for selectively closing one of the intake pipe sections tends to complicate the structure of the intake manifold.
Also, the selection valve for a dual port intake device is typically actuated by a vacuum actuator, and a negative pressure chamber for storing intake negative pressure is required. The need for a negative pressure chamber and the piping for communicating the negative pressure source to the negative pressure chamber and vacuum actuator also adds to the complexity of the intake device.